Although aggressive tumors often express abnormally low amounts of the cell cycle inhibitor p27'^'''^ (p27), this is almost never due to mutation of the p27 gene. Although the data supporting the importance of p27 as a prognosfic indicator are strong, it is ultimately the relationship that we and others are defining between p27 and response to specific breast cancer therapies that will expand the clinical ufility of this single marker. It is sometimes assumed that p27 is a surrogate for proliferation and that it is not prognostic or predictive beyond its role in inhibiting the cell cycle. In this project, we will build on basic findings that confirm p27 funcfion is more complicated and that its regulafion and cellular localization mediate tumor progression and cellular responsiveness to therapies directed at breast cancer cells. We will begin the translafion of these basic discoveries by evaluating the relationship between clinical outcome and p27 expression and cellular localization in human breast cancers of women with at least five years of follow-up for breast cancer mortality. Compelling evidence indicates that the cellular localizafion of p27 could be an indicator of response to two important breast cancer therapies: anti-estrogen and anti-HER2. To characterize the effect of an anfi-estrogen and trastuzumab on expression and localizafion of p27, we will assay protein levels in samples pre- and post-administration of these agents. These relafively small human studies will guide the evaluation of p27 in larger clinical trials that have the power to assess the ability of this protein to predict response to therapy. In parallel we will be investigating a new pathway, involving TRIM62, that may be responsible, at least in part, for the cytoplasmic expression of p27 in HER2+ human breast cancers. We propose three aims: Aim 1 will test the hypothesis that cytoplasmic p27 is a breast cancer oncogene affecting tumor initiation, invasion and metastasis. Aim 2 will test the hypothesis that regulation of p27 by TRIM62, a new p27 regulator, plays an essential role in HER2-dependent oncogenic transformation. Aim 3 will further test the hypothesis that increased cytoplasmic p27 is a prognosfic marker in human breast cancer, and ask whether cytoplasmic p27 predicts responsiveness to specific breast cancer therapeufics.